This invention relates to solid state lasers and particularly to diode-pumped solid state lasers that utilize phase conjugating Stimulated Raman Scattering (SRS) to generate the desired beam wavelengths suitable for microlithography and other applications.
The field of microlithography and the lasers needed to etch electronic microchips is burgeoning. At the present state of technology, one of the limiting parameters to smaller microchips and higher component density on such chips is the wavelength of the coherent light etching the components on the substrate. The quality of the chip circuitry and the economic viability of the production methods depend on a number of factors arising from the laser used for etching. These factors are the laser""s emission wavelength, the laser""s beam divergence and beam profile, the beam""s pulse duration, the beam""s pulse repetition rate, and the laser""s overall compactness and safety.
The laser""s emission wavelength determines the resolution that can be achieved on the individual chip. The shorter the laser""s emission wavelength, the higher the resolution that can be achieved. Since higher resolutions lead to higher component densities on the chip, this translates to more valuable chips.
The laser""s beam divergence and profile also affect the viability of the production method. A beam divergence that is too high, in conjunction with other beam parameters, renders a laser unsuitable for microlithographic applications. For the intended application, it is desirable to bring the laser""s beam divergence closer to the diffraction-limited, plane-wave, TEMOO single mode.
A high pulse repetition rate coupled with shorter pulse durations lowers the probability of damaging the manufactured components. Thus, it is desirable to have higher pulse repetition rates and, within certain limits, shorter pulse durations. The laser system""s overall compactness and work safety factor relate simply to the cost of chip fabrication plants.
Today""s mature excimer lasers can directly generate in the sub-200 nm wavelength range of the ultraviolet (UV) spectrum. However, these lasers have some unfortunate drawbacks. These excimer lasers, such as those outlined in U.S. Pat. No. 5,586,134 issued to Palash et al. and in U.S. Pat. No. 5,018,161 issued to Sandstrom et al., require the use of dangerous halogen materials such as xenon fluoride (XeF), krypton fluoride (KrF), argon fluoride (ArF) and others. The use of such gases necessitates large gas processing, storage and circulation technologies. Also, using such lasers in a fabrication plant requires changing the air inside the plant entirely every several hours for safety purposes. These lasers are bulky, complex, potentially hazardous, and expensive. Furthermore, these excimer lasers cannot generate at pulse repetition rates much higher than 1 kHz, and their beam mode is quite far from the desired TEM00 single mode.
On the other hand, solid state lasers do not suffer from the safety, repetition rate, and beam mode drawbacks of excimer lasers. Solid state lasers do not use dangerous gases and this eliminates the added expense and potential harm such gases can cause. Not only that, but solid state lasers can be pulsed at multi-kHz repetition rates that are much higher than that of excimer lasers. Furthermore, the beam quality of solid-state lasers can be close to the ultimately possible diffraction limit. However, solid state lasers do not directly emit in the desired UV range. The optical frequency, and thereby the wavelength, of the solid state laser""s emission must therefore be converted to the desired wavelength range to make them suitable for microlithography.
Such a conversion can be achieved by directly multiplying the optical frequency, a method known as harmonic generation. Harmonic generation is the most common method of frequency conversion (see e.g. W. Koechner. xe2x80x9cSolid-State Laser Engineeringxe2x80x9d, 3rd Edition, Springer-Verlag, Berlin, 1992). However, this method does not allow the generation of wavelengths other than the initial wavelength divided by an integer factor.
Fortunately, there are other methods that allow the production of UV emission from solid state lasers. One method that is utilized in U.S. Pat. No. 5,742,626 issued to Mead et al. is the use of an optical parametric oscillator (OPO). This method is based on a nonlinear process that includes producing two independent, separated beams from the initial single beam. This approach suffers from an unfortunate practical shortcoming. The method necessitates combining two separated beams to produce the desired UV output beam. A sum frequency generator would combine the OPO-produced beam with the fifth harmonic of the input laser beam. While combining such beams is theoretically and experimentally possible, achieving the necessary efficiency to make the method useful will be very difficult, if not practically impossible. One reason for this is that it is very difficult to achieve the precision required to align independent, separated beams with the necessary parallelism. Also, the pulses produced by the laser and by the OPO have different temporal shapes, further complicating the efficient mixing of the beams. Also, the OPO""s jitter and the substantially broadened spectrum preclude efficient mixing. The above reasons therefore render the method and apparatus of Mead et al. to be impractical.
Another method for frequency conversion that produces different wavelengths from fixed wavelength solid state lasers is the use of Stimulated Raman Scattering (SRS). Raman scattering is a process in which light is scattered at frequencies which are the sum and the difference between the incident frequency and the oscillation/vibration frequencies of the scattering material. When a scattering material is irradiated by a monochromatic light that has a frequency which does not correspond to any of the absorption lines of the material, frequency shifted components of the light can be detected in the scattered radiation. These shifted components have shifts independent of the irradiation frequency but characteristic of the scattering material. A laser beam, when passed through a scattering materialxe2x80x94or Raman mediumxe2x80x94produces shifted lines on the low frequency side and shifted lines on the high frequency side. The shifted lines on the low frequency side are called the Stokes-shifted lines and the shifted lines on the high frequency side are called the anti-Stokes shifted lines. Thus, laser light with frequency v scattered on a Raman medium consists of not only the initial frequency v but also the vxe2x88x92v1frequency (Stokes-shifted with a longer wavelength) and the v+v1 frequency (anti-Stokes shifted with a shorter wavelength) where v1 is characteristic of the Raman medium.
From the above, it can therefore be seen that SRS can produce different wavelengths of light given an input beam and a judicious choice of a Raman medium. The Raman medium could be gaseous, liquid or solid. The Raman threshold and other physical conditions for frequency shifting are very different for the three types of media and this leads to substantial differences in designs of gas, liquid and solid state Raman lasers. Most known practical designs of Raman lasers use gas Raman cells. U.S. Pat. No. 4,633,103 issued to Hyman et al. uses SRS in gas cells to produce yellow light for laser guide-star applications. U.S. Pat. No. 5,796,761 issued to Cheung et al. utilizes SRS in solid Raman cells to produce a laser beam in the visible range for similar applications. U.S. Pat. No. 5,673,281 issued to Byer also proposes using an SRS frequency conversion laser, this time using a solid SRS cell, for guide star applications. However, none of the methods above can convert laser light into the UV-range with a sufficient efficiency to be suitable for microlithography.
The patent by Hyman et al. describes a classic gas-cell based Raman laser where Raman cells are placed outside the laser resonator. Neither the wavelengths it generates nor its efficiency and beam divergence allow the design to be used for producing UV coherent light suitable for microlithographic applications.
Both Cheung""s and Byer""s concepts employ solid Raman cells. An essential part of both their designs is Raman unit installed inside the laser resonator. This design cannot be used for UV-microlithography because the placement of the Raman shifter does not allow the selection of the resonator transverse mode. Without this ability, the desirable beam divergence and beam quality cannot be achieved. Also, when the Raman cell is placed inside the resonator, there is a fluctuating delay (jitter) between the beginning of lasing and the emission of the SRS pulse. This leads to jitter instability in the frequency sum/difference crystal that precludes the achievement of microlithography required parameters.
The most significant drawback of all the above designs is the lack of a mechanism to completely eliminate forward scattering. As a rule, forward and backward scattering accompanies the generation of higher Stokes and anti-Stokes Raman spectral components. The presence of forward scattering lowers the efficiency of concentrating optical energy in one desirable line. A high concentration efficiency is especially important for the multi-step conversion of an input beam into the UV range. Additionally, in the presence of forward scattering, the generation of the single transverse beam-mode(TEM00)xe2x80x94that is very essential for microlithographic applicationsxe2x80x94is practically unachievable. Forward scattering is observed with all the above designs but with the intra-cavity SRS-cell the effect is stronger, and, despite the effort taken by Cheung, it cannot be eliminated totally.
Thus, while the concepts of Cheung, Hyman, and Byer are well suited for guide-star and other applications that do not require the best possible beam quality, none of them is readily applicable to microlithography.
To achieve an output beam in the TEM00-mode using SRS, the forward scattering must be entirely eliminated. This can be done by using the so called phase-conjugation geometry (see, e.g. V. Bespalov, and G. Pasmanik. xe2x80x9cNon-Linear Optics and Adaptive Systemsxe2x80x9d, Nova Science Publishers, New York, 1993). The phase conjugation principle, in simplest terms, is capable of inverting both the direction of propagation and the phase for each plane wave component of the incoming light beam. Without phase conjugation, the output of a Raman cell, given an input beam that is in the TEM00-mode, will be not be in the TEM00-mode. This change in mode is due to the presence of forward scattering. However, by using a phase conjugating geometry of the Raman cell, the frequency shifted beams will not have the undesirable forward scattering. Thus, if the input to a phase conjugating Raman cell is in the TEM00-mode, the output will also be a phase conjugated Stokes shifted beam in the TEM00-mode.
A combination of the frequency conversion methods outlined above and of the phase conjugation geometry would therefore produce a beam with the desired characteristics from a solid state laser. The frequency conversion methods would allow the generation of UV light while the phase conjugation geometry would eliminate the undesirable effect of Raman scatteringxe2x80x94forward scattering.
What is therefore required is a laser system that combines the frequency conversion methods outlined above with the concept of phase conjugation to produce laser light in the UV range with the characteristics that make that light suitable for microlithography.
The present invention overcomes the deficiencies identified in the prior art. The present invention provides a multiple stage solid state laser system producing an output beam having an output wavelength in the desirable ultraviolet range, the system comprising a first stage including an input beam stage producing an input beam, a plurality of further stages, each stage being chosen from the group comprising a phase conjugating frequency conversion stage, and a harmonic generation stage wherein each stage of the system is optically coupled to at least one other stage.
Preferably, the system includes a second stage comprising a first phase conjugating frequency conversion stage coupled to receive the input beam and producing a first phase conjugated Stokes shifted beam.
More preferably, the laser system includes a third stage comprising a harmonic generation stage coupled to receive the phase conjugated Stokes shifted beam and producing a Stokes harmonic beam that is a fixed harmonic of the first phase conjugated Stokes shifted beam.
Also preferably, the laser system includes a third stage comprising a harmonic generation stage coupled to receive the input beam and producing a harmonic beam that is a fixed harmonic of the input beam.
More preferably, the laser system includes a fourth stage comprising a second phase conjugating frequency conversion stage coupled to receive the harmonic beam and the first phase conjugated Stokes shifted beam and producing a phase conjugated anti-Stokes shifted beam.
Conveniently, the laser system includes a second stage comprising a harmonic generation stage coupled to receive the input beam and producing a harmonic beam that is a fixed harmonic of the input beam.
More conveniently, the laser system includes a third stage comprising a phase conjugating frequency conversion stage coupled to receive the harmonic beam and producing an output beam.
Also conveniently, the laser system includes a third stage comprising a second phase conjugating frequency conversion stage coupled to receive the input beam and producing a second phase conjugated Stokes shifted beam.
More conveniently, the laser system further includes a fourth stage comprising a harmonic generation stage coupled to receive the first phase conjugated Stokes shifted beam and producing a Stokes harmonic beam that is a fixed harmonic of the first phase conjugated Stokes shifted beam.
Most conveniently, the laser system further includes a fifth stage comprising a third phase conjugating frequency conversion stage coupled to receive the second phase conjugated Stokes shifted beam and the Stokes harmonic beam. The fifth stage also produces a phase conjugated anti-Stokes shifted beam.
Alternatively, the laser system further includes fourth and fifth stages
wherein the fourth stage comprises a first harmonic generation stage coupled to receive the first phase conjugated Stokes shifted beam and producing a first Stokes harmonic beam and
wherein the fifth stage comprises a second harmonic generation stage coupled to receive the second phase conjugated Stokes shifted beam and producing a second Stokes harmonic beam.
Furthermore, it is preferable that the laser system includes an output stage comprising a frequency conversion stage coupled to receive both the first and the second Stokes harmonic beams.
More preferably, the laser system further includes an output stage comprising a harmonic generation stage coupled to receive both the first and the second Stokes harmonic beams.
Also preferably, the frequency conversion stage of the laser system includes components chosen from the group comprising a phase conjugating SRS Stokes cell transmitting a phase conjugated Stokes shifted beam and a phase conjugating SRS anti-Stokes cell transmitting a phase conjugating anti-Stokes shifted beam.
In another embodiment, the invention provides a laser system comprising an input beam source and a plurality of optical devices for altering the frequency of a laser beam including at least one phase conjugating Raman medium cell and an optical harmonic generation apparatus wherein the input beam source is optically coupled to at least one of the plurality of optical devices for altering the frequency of a laser beam.
Yet another embodiment of the invention provides for a method of producing laser light having a wavelength in the ultraviolet range, the method comprising:
producing a pulsed input beam from an input beam source
passing the input beam through at least one initial wavelength altering apparatus to produce at least one first intermediate beam, the initial wavelength altering apparatus being chosen from the group comprising:
a phase conjugating Raman medium cell
an optical harmonic generation apparatus.
Preferably, the method further includes passing the at least one first intermediate beam through a secondary wavelength altering apparatus to produce an output beam, the secondary wavelength apparatus being chosen from the group comprising:
a phase conjugating Raman medium cell
an optical harmonic generation apparatus.
Alternatively, the method further includes passing the first intermediate beam through a secondary wavelength altering apparatus to produce a second intermediate beam, the secondary wavelength apparatus comprising a phase conjugating Raman medium cell producing a phase conjugated anti-Stokes shifted beam.
Conveniently, the method further includes the step of using at least one phase conjugating SRS Stokes cell to produce a phase conjugated Stokes shifted beam.
Preferably, the method further includes the step of using an optical harmonic generation apparatus to produce fixed harmonics of a laser beam.
Also preferably, the method further includes the step of using at least one phase conjugating SRS Anti-Stokes cell to produce a phase conjugated anti-Stokes shifted beam.
The advantages of the present invention are numerous. Diode-pumped lasers can be used with this invention to provide high repetition rates. Desirable UV laser light can be produced without complex gas handling mechanisms such as those used in excimer lasers. The use of SRS to shift the input beam frequency allows for a wide range of output beam wavelengths. By judiciously choosing a phase conjugating Raman medium with the desired characteristics and by generating the required harmonics, a legion of output beam wavelengths is available. Furthermore, the high conversion efficiency of the phase conjugating SRS process reduces the need for very high power input laser beams. This leads to further cost savings. Also, phase-conjugating SRS cells produce only backward scattered beams with no forward scattering, thereby helping to achieve the TEM00 quality of the output beam, if the input beam has the TEM00 quality.